This invention relates to a gravity-driven emergency core cooling system (ECCS) and also to a boiling water nuclear reactor (BWR) using the same.
Studies are being made of introducing equipment that operates statically (passively), utilizing natural force, without using dynamic (actively) devices such as pumps mainly in order to raise the simplicity of safety equipment and improve the reliability of boiling water nuclear reactors. Among others, gravity-driven emergency core cooling systems (ECCS) are known for pouring water into the nuclear reactor in case of an accident as shown in Japanese Patent Application Laid-Open Publication No. 02-115793 and Japanese Patent No. 2918353 the entire contents of which are incorporated herein by reference.
An ordinary boiling water nuclear reactor has a reactor containment vessel, which includes a dry well and a wet well. The dry well contains a rector pressure vessel. A pressure suppression pool is housed in the wet well and the water in the pool is to be used to fill the fuel exchange pool with water when suspending the operation of the reactor and replacing the core fuel.
When a loss-of-coolant accident (LOCA) occurs and the pipe connected to the reactor pressure vessel is ruptured in the inside of the dry well, the water level in the reactor pressure vessel falls. Then, as the signal indicating the water level in the reactor pressure vessel or the pressure level of the dry well is detected, the isolation valve arranged at the gravity-driven water injection pipe connected to a gravity-driven water injection pool is opened to inject cooling water into the reactor pressure vessel by means of the gravity-driven ECCS.
The injected cooling water gradually raises the water level in the reactor pressure vessel. As the water level reaches the ruptured part of the pipe, water spills and falls into the dry well to submerge a lower part of the dry well which is referred to as a “dry well lower region” in water. Therefore, a gravity-driven water injection pool is required to contain a large volume of water of about 1,500 m3.
The gravity-driven water injection pool is held in communication with the dry well and its performance of injecting water into the reactor pressure vessel is determined by the water head of gravity-driven water injection pool. Therefore, the pressure difference between the inside of the reactor pressure vessel and that of the dry well needs to be minimized by opening the pressure-reducing valve before opening the isolation valve arranged at gravity-driven water injection pipe. While the steam generated continuously by the decay heat coming from the core is discharged partly into the dry well by way of the pressure-reducing valve, it is introduced to and condensed by the heat exchanger of the static containment vessel cooling system so that the internal pressure of the reactor containment vessel will not be raised excessively. In this way, when a LOCA occurs, the core and the reactor containment vessel are effectively cooled and held safe and sound.
On the other hand, when the isolation valve and/or the pressure-reducing valve of the gravity-driven water injection pipe become unopenable for some reason or another, there arises a situation where the core can no longer be cooled. Then, the core can become damaged and molten. If such is the case, the molten core melts the reactor pressure vessel and runs through the latter to fall into and settle in the dry well lower region.
If the molten core is not cooled and held to a hot condition, it reacts with the bottom concrete of the dry well lower region to produce noncondensible gas, which can raise the internal pressure of the reactor containment vessel. To cope with such a situation, a piping arrangement may be adapted such that by detecting the temperature rise of the dry well lower region, the injection pipe of the gravity-driven water injection pool extending to the lower part of the dry well is opened to inject cooling water onto and cool an upper part of the molten core.
Besides a LOCA, an anticipated transient without scram (ATWS) can take place to threaten the safety of nuclear reactor. This is an incident where the attempt to insert the control rods fails partly or totally although a scram is required for the nuclear reactor in order to protect the core and the reactor containment vessel when an accident of total loss of water feed to the nuclear reactor or an incident of a total closure of the main steam isolation valve occurs.
In an ATWS where the trouble proceeds with high internal pressure of the reactor, accompanying isolation of the reactor, a large quantity of reactor coolant is discharged in the form of steam from the main steam safety relief valve into the wet well to raise the internal pressure of the reactor containment vessel. Therefore, it is essential to maintain the soundness of the core and the reactor containment vessel by appropriately maintaining the water level in the reactor and forcing the reactor to quickly move into a subcritical state by injecting boric acid aqueous solution from the boric acid aqueous solution injection tank. The boric acid aqueous solution injection tank contains only about 20 m3 of boric acid aqueous solution that can put the core back into a subcritical state when mixed with the cooling water in the reactor pressure vessel and hence is very small if compared with the gravity-driven water injection pool.
Assume a situation where an ATWS incident takes place and the pressure-reducing valve is opened so that water is injected by the gravity-driven ECCS. In such a situation, there arises a possibility that the injection of boric acid aqueous solution has been completed before the injection of water by means of the gravity-driven ECCS is started to make it difficult to maintain the subcritical state so that the core may be plunged back into a critical state because the boric acid is diluted by about 1,500 m3 cold water injected by the gravity-driven ECCS. On the other hand, the safety of the reactor can be made more reliable when the boric acid concentration is held to a certain high level in such a situation.
In view of the above-identified circumstances, it is therefore the object of the present invention to make it possible to secure the safety of a reactor by reliably maintaining a subcritical state even when the gravity-driven ECCS is actuated when an ATWS takes place in a boiling water nuclear reactor.